1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium, a process of production thereof, and a magnetic storage equipped therewith. The perpendicular magnetic recording medium has a magnetic layer composed of columnar magnetic crystal grains whose principal component is cobalt, and it is characterized by reduced thermal decay. The magnetic storage has a recording density in excess of 50 Gbit/in2.
2. Description of the Related Arts
There is an increasing demand for higher recording density in magnetic storage from the standpoint of increasing the storage capacity, miniaturizing the apparatus, and reducing the number of parts. The existing magnetic recording medium is based on longitudinal magnetic recording. It records information by means of mutually opposed domains (recording bits) which are magnetized in the direction parallel to the surface of the substrate. For a longitudinal magnetic recording medium to be capable of high-density recording, it should have a low noise level. One effective way of noise reduction is finely reducing in size of crystal grains and even out particle diameters (or reduce the dispersion of particle diameters). This is exemplified by the invention (disclosed in Japanese Patent Laid-open No. 269548/1998) relating to a longitudinal recording medium which specifies for noise reduction the optimum particle diameter and the optimum dispersion of particle diameters.
Increasing the recording density in longitudinal magnetic recording will have a limit because of the necessity for more finely reduced crystal grains than before. However, extremely small crystal grains encounter problems with thermal decay. In other words, their magnetization for recording is decayed by even such small thermal energy as generated at room temperature. In order to address the problem with thermal decay, there has been proposed the perpendicular magnetic recording system, which is attracting great attention. It is essentially suitable for high-density recording by virtue of its property that the thermal stability of magnetization improves as the recording density increases.
The recording medium for perpendicular magnetization under wide study is one which has a magnetic layer composed of practically columnar crystal grains, with their (00.1) plane oriented nearly parallel to the surface of the substrate for their magnetic anisotropy in the direction perpendicular to the substrate. The most widely studied recording medium with a CoCr alloy magnetic film has a coercive force of about 3000 Oe (equivalent to approximately 79.7 A/m in SI unit). In order to put to practical use the magnetic recording medium composed mainly of cobalt, the present inventors carried out extensive studies, which led to an important finding that a magnetic film of cobalt alloy with the c-axis (or the (00.1) direction) oriented perpendicular to the surface of the substrate has a stacking fault density which is two to three times higher than that of cobalt alloy magnetic film with longitudinal orientation.
The perpendicular magnetic recording also needs a magnetic recording medium with low noise and high thermal stability. No matter whether it is of longitudinal recording type or perpendicular recording type, the magnetic layer with many stacking faults will be poor in magnetic anisotropy, coercivity, and thermal stability. For high thermal stability of recording magnetization, it is necessary to reduce the stacking fault density in the magnetic film.
The major cause of stacking faults is ingression of a plane corresponding to the fcc-like structure into the hcp structure. It is believed that the stacking fault density will decrease if the magnetic film is formed at a low temperature desirable for the hcp structure to be stable. However, the present inventors"" elucidation suggests that the magnetic film formed at low temperatures decreases in stacking fault density but does not increase in coercive force because crystal grains constituting the magnetic film have a broad distribution of particle diameters. Raising the film-forming temperature to reduce the dispersion of particle diameters increases the stacking fault density, with coercivity remaining low.
The longitudinal magnetic recording medium is inherently little subject to stacking faults because crystal grains constituting the magnetic layer are epitaxially grown on the underlayer such that the c-axis of magnetic grain is longitudinally oriented. Epitaxial growth takes place in the direction toward the most stable energy state. Thus, there is almost no possibility that epitaxial growth brings about an unstable energy state due to ingression of a crystal phase different from that of the fcc structure.
It was found from the present inventors"" investigation that the stacking fault density in the longitudinal magnetic recording medium formed at about 250xc2x0 C. is one half to one-third of that in the perpendicular magnetic recording medium. It was also found that the dispersion of particle diameters in the longitudinal magnetic recording medium is about 0.3 to 0.4, which is determined almost entirely by the dispersion of particle diameters in the underlying film of chromium alloy and which does not depend on the film-forming temperature.
The film-forming temperature affects the stacking fault density and the dispersion of particle diameters as shown in FIGS. 1 and 2 respectively. It is to be noted from FIG. 1 that the stacking fault density decreases as the film-forming temperature decreases. It is also to be noted from FIG. 2 that the dispersion of particle diameters increases as the film-forming temperature decreases. This suggests that it is difficult to have both of a low stacking fault density and a low dispersion of particle diameters. Such an antinomic relation of the film-forming temperature with the stacking fault density and the dispersion of particle diameters has never been anticipated in the technology of longitudinal magnetic recording medium.
It is an object of the present invention to provide a perpendicular magnetic recording medium which is made of conventional CoCr alloy as a magnetic material and yet has good thermal stability owing to adequate control over the stacking fault density and the dispersion of particle diameters. Being made of a conventional magnetic material, the recording medium is economically advantageous.
It is another object of the present invention to provide a method of adequately controlling the stacking fault density and the dispersion of particle diameters.
The objects of the present are achieved by controlling the stacking fault density (R) and the dispersion of particle diameters (xcex94D/ less than D greater than ) of the magnetic film, which is composed of magnetic crystal grains whose principal component is cobalt, such that the product of xcex94D/ less than D greater than xc3x97R is no larger than 0.02.
The stacking fault density relates with coercive force as shown in FIG. 3. Incidentally, dotted lines in FIG. 3 are contour lines for some values of the dispersion of particle diameters which are determined by the coercive force and the stacking fault density. It is noted from FIG. 3 that the coercive force increases as the stacking fault density decreases and the coercive force slightly decreases as the stacking fault density decreases further (due to increase in the dispersion of particle diameters).
FIG. 4 shows the contour lines of coercive force which are determined by the stacking fault density and the dispersion of particle diameters. It is noted that the coercive force no smaller than 4000 Oe is obtained in the area (under the thick line) in which the product of the stacking fault density and the dispersion of particle diameters is no larger than 0.02. The coercive force of 4000 Oe is necessary to suppress thermal decay. With a value of coercive force no smaller than 4000 Oe, the magnetic recording medium has a value of Kuxc2x7V/kxc2x7T no smaller than 60 which is necessary to ensure thermal stability for recording magnetization. This value is a parameter to indicate resistance to thermal decay. (In Kuxc2x7V/kxc2x7T, Ku denotes a magnetic anisotropy energy possessed by crystal grains, V denotes a volume of crystal grains, k denotes the Boltzmann constant, and T denotes an absolute temperature.) According to the present invention, this parameter Kuxc2x7V/kxc2x7T should have a value no smaller than 60; otherwise, the magnetic recording medium is not of practical use because it remarkably decreases in the amount of recorded magnetization. To achieve this object, the magnetic recording medium should have a coercive force no smaller than 4000 Oe in view of the fact that the existing cobalt-based magnetic material has a value of about 0.38 T (tesla) for saturation magnetization and the magnetic layer has a thickness of 18 nm and the magnetic crystal grains have an average particle diameter of about 12 nm (both attainable by the present technology).
One way to decrease both the stacking fault density and the dispersion of particle diameters is to form the magnetic film at a temperature no lower than about 250xc2x0 C. and then anneal the resulting magnetic film. Film forming at a high temperature increases the stacking fault density, but annealing decreases the stacking fault density.
The following is a probable reason why the stacking fault density is decreased by annealing. The magnetic film, which is formed usually by sputtering, is subject to stacking faults because sputtering, which is a non-equilibrium process, does not permit atoms constituting the magnetic film to diffuse completely into the film. In other words, sputtering fails to arrange atoms at energy-stable positions (or hcp lattices) but results in stacking faults. Annealing after film formation moves atoms to the hcp lattice positions.
The present invention is also directed to a magnetic recording/reading unit which is constructed of the above-mentioned magnetic recording medium, a drive mechanism to convey the recording medium, and a magnetic head for record writing/reading, which is a component capable of producing a high magneto-resistive effect. The magnetic recording/reading unit has a recording density in excess of 50 Gbit/in2. The magnetic head should preferably be one which utilizes the giant magneto-resistive effect, spin-valve effect, or tunneling magneto-resistive effect.